Re: CCL:Orbitals

2003 May 28
 Are MOs physically real? This is a meaningful question only if there is some
 experiment or observation that could provide an answer _yes_ or _no_.  Is there,
 at least in principle, such an experiment or observation?
 As J S-L points out, MOs are one-electron functions; does this mean that for
 hydrogenlike atoms they _do_ correspond to physical reality?
 If MOs have no physical reality for multielectron species, why (a) is Koopmans'
 theorem useful,  why (b) do photoelectron spectra match the predictions of MO
 energy-level diagrams, and why (c) does the Hueckel 4n+2 rule, which is based on
 MO diagrams, work? Of course, it is probably possible to formulate an MO-free
 electronic molecular theory that leads to the same predictions a-c, but I
 suspect
 that in some sense (but what sense?) MOs exist--occupied MOs; the meaning of a
 virtual MO is harder to see.
 E. Lewars
 =====
 Jens Spanget-Larsen wrote:
 > Mikael Johansson:
 >
 > > On Tue, 27 May 2003, Jens Spanget-Larsen wrote:
 > >
 > > > And in principle, MOs are not physical quantities; they are model
 > > > constructions, and as such they have no physical reality and they
 > > > cannot be observed experimentally.
 > >
 > > Well, there is a lot of discussion in the literature on this topic.
 > > A few good examples, biased towards my thinking, could be:
 > >
 > > [1] Stowasser and Hoffmann, "What Do the Kohn-Sham Orbitals and
 > >     Eigenvalues Mean?", J.Am.Chem.Soc. 121 (1999) 3414-3420.
 > > [2] Baerends, Theor.Chem.Acc. 103 (2000) 265-269.
 > >
 > > Have a nice day,
 > >     Mikael J.
 > >     http://www.helsinki.fi/~mpjohans/
 >
 > Dear Mikael!
 >
 > Yes, it is true, there has been some discussion on the status of the
 > MO concept. But to me, there can be no doubt: In principle, orbitals
 > are one-electron wavefunctions, and for a many-electron system, they
 > do by definition NOT correspond to physical reality. However, they
 > are extremely useful models, as we all know. The usefulness of the MO
 > concept in rationalizing a lot of chemistry and spectroscopy
 > sometimes leads people to think of MOs as physical quantities, rather
 > than simplified models. Do you think my point of view is erroneous?
 >
 > Yours, Jens >--<
 >
 > ------- End of forwarded message -------
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